BMED 3106Integrated Body Systems IIIRenal System, Fluids, & ElectrolytesDepartment of Health & Biomedical Sciences

Developmental Anatomy of the Renal system&Microscopic Anatomy of the Renal system

The intermediate mesoderm forms a longitudinal elevation along the dorsal body wall called the urogenital ridge. A portion of the urogenital ridge forms the nephrogenic cord, which gives rise to the urinary system.

The nephrogenic cord develops into three sets of nephric structures: Pronephros Mesonephros Metanephros

The pronephros develops by the differentiation of mesoderm within the nephrogenic cord to form pronephric tubules and the pronephric duct. The pronephros is a transitory structure that regresses completely by week 5)

The mesonephros develops by the differentiation of mesoderm within the nephrogenic cord to form mesonephric tubules and the mesonephric duct. The mesonephros is the middle nephric structure and is a partially transitory structure. Most of the mesonephric tubules regress, but the mesonephric duct persist and opens into the urogenital sinus).

The metanephros develops from an outgrowth of the mesonephric duct (called the ureteric bud) and from the metanephric mesoderm. The metanephros develops into the definitive adult kidney.

Development of the metanephrosThe ureteric bud is an outgrowth of the mesonephric duct.The ureteric bud initially penetrates the metanephric mesoderm and then undergoes repeated branching to form the ureters, renal pelvis, major calyces, minor calyces, and collecting ducts.Development of the collecting system

Development of the metanephrosDevelopment of the nephronMetanephric mesoderm differentiate into metanephric vesicles which later give rise to primitive S-shaped renal tubules, which are critical to nephron formation. The S-shaped tubules differentiate into the proximal and distal convoluted tubules, loop of Henle, and Bowmans capsule. Tufts of capillaries (glomeruli) protrude into the Bowmans capsule.

Relative ascent of the kidneysFetal metanephros is located at vertebral level S1-S2, whereas the definitive adult kidney is located at vertebral level T12-L3. The change in location results from a disproportionate growth of the embryo caudal to the metanephros.During the relative ascent of the kidneys, they will receive blood supply from arteries at progressively higher levels until the definitive renal arteries develop. Arteries formed during the ascent may persist and are called supernumerary arteries.

Development of the Urinary bladderThe urinary bladder is formed from the upper portion of the urogenital sinus, which is continuous with the allantois.The allantois becomes a fibrous cord called the urachus.

Development of the male urethraIn the male the lower portion of the urogenital sinus develops into the penile urethra. The prostate gland is formed by buds from the urethra, and seminal vesicles are formed by budding from the ductus deferens.

Development of female urethraThe female urethra is formed from the lower portion of the urogenital sinus. The female urethra develops endodermal outgrowths into the surrounding mesoderm to form the urethral glands and paraurethral glands.

Histology: The Urinary (Renal) systemThe urinary system consists of the paired kidneys and ureters, urinary bladder, and the urethra.

Urinary system helps to maintain homeostasis by a complex combination of functions: Filtration Selective reabsorption Excretion

KidneyEach kidney is bean-shaped, with a concave hilum where the ureter and renal vasculature enter. The ureter divides and subdivides into several major and minor calyces, around which is located the renal sinus containing adipose tissue.

Blood supply to the kidney

Each kidney contains about 1 1.4 million functional units known as nephrons. Major divisions of the nephron are: Renal corpuscle PCT Loop of Henle DCT

Collecting duct

The Renal corpuscleThe renal corpuscle is a small mass of capillaries (glomerulus) housed within a capsule (Bowmans capsule). The internal lining of the capsule is composed of specialized epithelial cells called podocytes, which cover each capillary, forming filtration slits between interdigitating processes called pedicels.

A section of one glomerulus shows many capillaries and the closely associated cells of the renal corpuscles internal visceral layer. The thick basement membrane of these glomerulus capillaries contains much type IV collagen and is visible around the cut capillaries (arrows). Also shown are the simple squamous external parietal layer of the capsule and the vascular pole where the arterioles enter the corpuscle and the macula densa (arrowhead) is located. X400.PSH

Glomerular filtration membrane

The Glomerular filtration membrane consists of three layers: Fenestrated capillary endothelium Glomerular basement membrane Filtration slits between podocytes processes.

PC- PodocytesFiltration slits arrowsC capillary BL basement membraneUS capsular (urinary) spaceE capillary endothelial cell

In addition to capillary endothelial cells and podocytes, renal corpuscles also contain mesangial cells located between capillaries and surrounded by extracellular matrix.Functions of mesangial cells: Physical support and contraction Phagocytosis Secretion of cytokines and prostaglandins.MC mesangial cellMM mesangial matrixBM basement membraneP podocytesPD pedicelsE erythrocyte & L leukocyteEC endothelial cell

Proximal convoluted tubuleThe micrograph shows the continuity at the renal corpuscles tubular pole (TP) between the simple cuboidal epithelium of a PCT (P) and the simple squamous epithelium of the capsules parietal layer. The capsular space (U) between the parietal layer and the glomerulus (G) drains into the lumen of de PCT. The lumen of the PCT appears filled, due to the long microvilli of the brush border and proteins bound to this structure. By contrast, the lumens of the DCT (D) appear empty, lacking a brush border and protein.

Renal medulla: Nephron loops and Collecting ductsA micrograph of a medullary pyramid cut transversely shows closely packed cross sections of the many nephron loops thin descending limbs (T) and thick ascending limbs (A), intermingled will parallel vasa recta capillaries (C) and collecting ducts (CD)All these structures are embedded in the interstitium (I) which helps maintain the osmolarity gradient required to concentrate urine and conserve body water.

The Juxtaglomerular apparatusMicrograph shows that the JGA forms at the point of contact between the DCT (D) and the vascular pole of its glomerulus. At that point cells of the DCT become columnar as a thickened region called macula densa (MD). Smooth muscle cells of the afferent arteriole (AA) are converted from a contractile to a secretory morphology as juxtaglomerular granule cells (JG). Also present are lacis cells (L). Also visible is the efferent arteriole (EA)

Collecting ductsLongitudinally sectioned renal pyramid showing two collecting ducts (CD) and their distinct lateral cell boundaries (arrows), with interstitial connective tissue. X400. H&E

Collecting ducts adjust the ionic composition of urine in their lumens and allow increased water reabsorption from this urine when fluid levels in the body are low. This occur under the influence of ADH.

Collecting tubules and ducts are composed mainly of Principal cells (few organelles and scanty microvilli). Scattered among the principal cells are variably darker intercalated cells with more abundant mitochondria which help regulate the acid-base balance by secreting H+ and absorbing HCO3-.Transverse section of collecting ducts (CD) with vasa recta capillaries (VR) in the interstitium.Weakly stained duct principal cells are initially cuboidal and become increasingly columnar along the ducts.

Renal papilla, collecting ducts, and minor calyxA sagittal section of the renal papilla (RP) shows numerous collecting ducts converging at the end of the renal papilla (RP). The mucosa of the calyx contains dense CT stained blue here and adipose tissue (A).Inset: An enlarged area shows the columnar epithelium of the collecting ducts (CD), the interstitium (I) and thin limbs (T) and the protective urothelium (U) that lines the minor calyx.

UretersCross section of the ureter shows a characteristic pattern of longitudinal folded mucosa, surrounded by a thick muscularis that moves urine by peristalsis. The lamina propria is lined by a unique transitional epithelium resistant to the potentially deleterious effects of contact with hypertonic urine.

Urinary bladderThe bladder is a muscular sac that is expandable as it fills with urine.Layers:Mucosa (transitional epithelium and lamina propria)SubmucosaMuscularis layer (Detrusor muscle)

a- in the neck of the bladder, near the urethra, the wall shows four layers: mucosa with urothelium (U) and lamina propria (LP); the thin submucosa (S); inner, middle, and outer layers of smooth muscle (IL,ML,OL), and the adventitia (A). X15 H&Eb- when the bladder is empty, the mucosa is highly folded and the urothelium has bulbous umbrella cells. X250.PSHc- when the bladder is full, the mucosa is pulled smooth, the urothelium is thinner, and the umbrella cells are flatter. X250.H&E

UrethraThe urethra is a fibromuscular tube that carries urine from the bladder to the exterior.A transverse section shows that the mucosa has large longitudinal folds around the lumen (L). X50 H&E.A higher magnification of the urethral epithelium is shown in this micrograph. The thick epithelium is stratified columnar in some areas and pseudostratified columnar elsewhere, but becomes stratified squamous at the distal end of the urethra. X250. H&E.

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